btoa()

The btoa() method creates a Base64-encoded ASCII string from a binary string (i.e., a String object in which each character in the string is treated as a byte of binary data).

You can use this method to encode data which may otherwise cause communication problems, transmit it, then use the atob() method to decode the data again. For example, you can encode control characters such as ASCII values 0 through 31.

stringToEncode

The binary string to encode.

An ASCII string containing the Base64 representation of stringToEncode.

InvalidCharacterError DOMException

The string contained a character that did not fit in a single byte. See "Unicode strings" below for more detail.

The btoa() function takes a JavaScript string as a parameter. In JavaScript strings are represented using the UTF-16 character encoding: in this encoding, strings are represented as a sequence of 16-bit (2 byte) units. Every ASCII character fits into the first byte of one of these units, but many other characters don't.

Base64, by design, expects binary data as its input. In terms of JavaScript strings, this means strings in which each character occupies only one byte. So if you pass a string into btoa() containing characters that occupy more than one byte, you will get an error, because this is not considered binary data:

const ok = "a";
console.log(ok.codePointAt(0).toString(16)); //   61: occupies < 1 byte

const notOK = "✓"
console.log(notOK.codePointAt(0).toString(16)); // 2713: occupies > 1 byte

console.log(btoa(ok));    // YQ==
console.log(btoa(notOK)); // error

If you need to encode Unicode text as ASCII using btoa(), one option is to convert the string such that each 16-bit unit occupies only one byte. For example:

// convert a Unicode string to a string in which
// each 16-bit unit occupies only one byte
function toBinary(string) {
  const codeUnits = new Uint16Array(string.length);
  for (let i = 0; i < codeUnits.length; i++) {
    codeUnits[i] = string.charCodeAt(i);
  }
  const charCodes = new Uint8Array(codeUnits.buffer);
  let result = '';
  for (let i = 0; i < charCodes.byteLength; i++) {
    result += String.fromCharCode(charCodes[i]);
  }
  return result;
}

// a string that contains characters occupying > 1 byte
const myString = "☸☹☺☻☼☾☿";

const converted = toBinary(myString);
const encoded = btoa(converted);
console.log(encoded);                 // OCY5JjomOyY8Jj4mPyY=

If you do this, of course you'll have to reverse the conversion on the decoded string:

function fromBinary(binary) {
  const bytes = new Uint8Array(binary.length);
  for (let i = 0; i < bytes.length; i++) {
    bytes[i] = binary.charCodeAt(i);
  }
  const charCodes = new Uint16Array(bytes.buffer);
  let result = '';
  for (let i = 0; i < charCodes.length; i++) {
    result += String.fromCharCode(charCodes[i]);
  }
  return result;
}

const decoded = atob(encoded);
const original = fromBinary(decoded);
console.log(original);                // ☸☹☺☻☼☾☿

See also the example utf8_to_b64 and b64_to_utf8 functions in the Solution #1 – escaping the string before encoding it section of the Base64 glossary entry.

• A polyfill of btoa is available in core-js
• data URIs
• atob()
• Base64
• Polyfill